1. Field of the Invention
The present invention may involve reducing or eliminating aberrations produced by an irregularly shaped cornea surface during examination or treatment of the eye. This invention also relates to apparatus and methods for stabilizing the position of the eye during examination or treatment procedures.
2. Discussion of Background
The cornea is a transparent tissue that not only allows transmission of light into the eye, but also provides most of the optical power for focusing light on the retina, the image sensing portion of the eye. The crystalline lens, located approximately 4 mm behind the cornea, provides the additional optical power needed to focus the image precisely at the retina.
Corneas are aspheric, in that the curvature is greatest at the center and less toward the periphery. Most corneas also have some astigmatism, i.e., greater curvature in one azimuthal direction than in the perpendicular direction. The irregular shape of the eye affects the path of light through the eye. This aberration in the light path may be corrected by spectacles or contact lenses.
Further, almost all corneas, when measured carefully, have some degree of irregular astigmatism. In irregular astigmatism, irregularities in the shape of the cornea cannot be optically corrected by a combination of spherical and optical elements.
One way to reduce the effects of astigmatism involves use of contact lenses. A rigid, i.e., non-flexible, contact lens will generally have a layer of tears between the lens and the cornea, the thickness of which will vary from center to edge. The layer of tears improves the image which reaches the retina because the refractive index of the tears (1.336) is closer to the refractive index of the cornea (1.376) than the refractive index of air (1.0). Thus, when a rigid contact lens is used, variations in cornea topography have less effect on light rays than they would when the cornea is in air. As a result, the optical effects of corneal irregularity are significantly reduced. However, if the cornea astigmatism is sufficiently large, there will be residual astigmatism due in part to the fact that the refractive index of the tear layer does not exactly match that of the cornea. Further, there may be astigmatism in the crystalline lens or other irregularity in the crystalline lens.
As a result, for many years, contact lenses have been used with patients having irregular corneas, particularly corneas having high levels of astigmatism. A hard contact lens with a spherical back surface is prescribed, and the tear layer between the back surface and the cornea reduces the aberrations of the cornea by a factor of about: ##EQU1## That is, the aberrating effect of an irregular cornea is reduced to about 10% of that for the cornea in air.
Taking into consideration the known methods for correcting vision, there are several known techniques for facilitating the treatment or examination of the eye. Biomicroscopes, also known as slit lamps, are often used with a diagnostic contact lens which is hand-held against the cornea, utilizing a viscous liquid such as a methylcellulose solution to form an optical coupling to the cornea.
This procedure for using diagnostic contact lenses reduces the aberrations that would be produced by the same cornea in air. The layer of methylcellulose solution reduces the effects of astigmatism because the refractive index of the methylcellulose solution (1.337) is closer to the refractive index of the cornea (1.376) than the refractive index of air (1.0). As a result, variations in cornea topography have less effect on light rays than they would when the cornea is in air.
Examples of diagnostic contact lenses which are used in conjunction with biomicroscopes include diagnostic contact lenses made by Ocular Instruments, Inc. and Volk Optical Co. These contact lenses are hand-held lenses with concave front surfaces to contact the cornea, generally used with a viscous liquid such as methylcellulose solution. Some of these lenses are gonio-lenses which include inclined mirror surfaces that allow examination of various regions of the retina and the region called the angle of the anterior chamber. FIG. 5 of WILENSKY, "Optics of Gonioscopy", Clinical Ophthalmology, Vol. 1 (1990), shows how the gonio-lenses contact the eye, and how a ray of light travels from the angle recess of the eye to the mirror surface and then out of the lens.
U.S. Pat. No. 5,359,373 to KOESTER et al., the disclosure of which is herein incorporated by reference in its entirety, discloses a contact lens element with a flat front surface for contacting the cornea to stabilize the longitudinal position of the cornea. Because the contact lens element flattens the portion of the cornea against which it is pressed, the contact lens element helps to reduce aberrations caused by the normal, unflattened shape of the cornea. Column 5 of this document discloses that small variations from flatness can be utilized, and discloses that if the surface is concave with a radius of curvature less than that of the cornea, it is possible to trap air bubbles in the tear layer between the element and the cornea, thereby disrupting the optical continuity of the system. Column 7 of this document indicates that while a concave contact surface might have optical advantages, it could possibly cause a greater distortion of the cornea if it is not precisely aligned with the axis of the cornea. The flat contact lens element, while useful for examining the cornea, is not suitable for examination of the crystalline lens and details at other depths within the eye. The flattening of the cornea has the effect of causing wrinkles, which show up as a corneal mosaic as discussed in AURAN et al., "Wide Field Scanning Slit in vivo Confocal Microscopy of Flattening-Induced Corneal Bands and Ridges", Scanning, Vol. 16, pp. 182-86 (1994). These wrinkles produce inhomogeneities in the optical path through the cornea and have been observed to result in a blurring of the retinal image during examination at high magnification.
KOESTER et al., "Clinical Microscopy of the Cornea Utilizing Optical Sectioning and a High-Numerical-Aperture Objective", J. Opt. Soc. Am. A, Vol. 10, No. 7 (July 1993), the disclosure of which is herein incorporated by reference in its entirety, discloses contact lens elements similar to those disclosed in U.S. Pat. No. 5,359,373 to KOESTER et al. This article discloses that a slightly concave surface can be used for the contact lens element, as long as the radius of curvature is greater than that of the cornea, to reduce the possibility of trapping air bubbles between the element and the cornea.